It's sold under the name アセトアミノフェン (acetaminophen) in Japan.
http://www.survivingnjapan.com/2011/01/how-to-find-tylenol-i...
http://www.survivingnjapan.com/2011/01/how-to-find-tylenol-i...
It's heavily advertised and available in every drug store in Japan (you don't even need to go to pharmacies!).